1. Field of the Invention
The present invention relates to a low pass filter. More particularly, the present invention relates to a low pass filter in which a low frequency cut filter or a low frequency pass filter having a low capacitance can be implemented in a chip using an active resistor and an amplifier.
2. Description of the Prior Art
Recently, according to a growing interest for the radio communication, there have been made much efforts to develop low price handy terminals. One point to be considered in developing such integrated circuits for use in communication is that large direct current (DC) offset is occurred in output by means of filter and mixer. Thus, trimming circuit is used generally to remove such DC offset. If the offset becomes large in CMOS circuits, however, an amount of bits to be trimmed becomes large, so that it takes a lot of time in test, and values of DC offset vary in the course of time.
Therefore, new solution is to minimize effects of DC offset by using a low frequency filter. Since such a low frequency filter has to process signals under several hundred hertz normally, however, it requires very large resistor and capacitor, thereby making integration difficult. Nevertheless, if the resistor and capacitor are located outside the chip, it makes low costs difficult.
Prior art low frequency filters shown in FIGS. 1A and 1B form low frequency filters having a cutoff frequency f.sub.3db at low frequency using large resistor or large capacitor located outside the chip.
FIG. 1A shows a fundamental structure of general low frequency cut filter comprised of a capactor C1 and a resistor R1, and FIG. 1B shows a fundamental structure of general low frequency pass filter comprised of a resistor R2 and a capactor C2. The low frequency cut filter cuts off low frequency components including DC by means of the capacitor C1. In this case, the cutoff frequency f.sub.3db of the low frequency cut filter can be expressed as a following equation 1: EQU f.sub.3db =1/2.pi.R1.multidot.C1 (1)
If making the cutoff frequency f.sub.3db be 100 Hz, the value of R1.multidot.C1 has to be about 1.6E-3. It is assumed that a maximum capacitor which can be formed within the integrated circuit is 30 pF, it should become R=53M.OMEGA., which requires very large region within the integrated circuit. Where in general CMOS process, a sheet resistance is 50.OMEGA. and a width of the sheet resistance is 2 .mu.m, a length L necessary for making R=53M.OMEGA. resistance is as follows: EQU L=(53M.OMEGA./50.OMEGA.).times.2 .mu.m=2120 mm (2)
As seen in equation 2, since such a resistance is very large, it is not proper for integration-circuit. In the contary, it is next to impossible that the resistor is decreased and the capacitor is increased. This can be applied to the low frequency pass filter equally.